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Old 28th September 2010, 06:06 PM   #1
sycorax is offline sycorax  Netherlands
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Default How to test interstage transformers?

I'am experimenting with interstage transformers for the first time and I'am getting strange results. at first I was using a simple amp with a D3A. Now I have made this test setup.

This is how I test:

To the left is a generator with a Ri of 8K and to the left is a load of 8K and the audio analyser is parallel with the load.

What is going wrong?


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Old 28th September 2010, 06:08 PM   #2
SY is offline SY  United States
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Source impedance too high, load impedance too low.
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Old 28th September 2010, 06:16 PM   #3
sycorax is offline sycorax  Netherlands
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Quick response, I forget to mention, it is a 1:1 transformer. Z generator is 8K and Z load should be also 8K please correct me if I'am wrong.
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Old 28th September 2010, 06:25 PM   #4
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That's true for maximum power transfer- but that's not the point of most ITs. If you're coming off the plate of an undegenerated D3a, the source impedance is likely to be 1-2k. Load is whatever grid resistance and input capacitance you have in the next stage.
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Old 28th September 2010, 06:27 PM   #5
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What transformer is it.

Pieter (also north of the Netherlands)
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Old 28th September 2010, 07:08 PM   #6
sycorax is offline sycorax  Netherlands
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I have done the same measurement with a source impedance of 1K and a load of 100K.

Nearly the same result.

At first I was using the D3A with the interstage to drive a GM70 tube also with this results. So I'am doing something fundamentally wrong or there is something wrong with the transformer.

The transformer is made by a respected Dutch transformer maker. No need to mention names. He is willing to investigate the matter but I have to be sure that it is not my stupidity first:-)
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Old 28th September 2010, 07:20 PM   #7
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Reverse the secondary and measure again.
When it is a bifilar wound IT from my respected Dutch colleage you probably have an anti-phase connection.
Bifilar wound transformers have excellent bandwidth when connected right, and lousy bandwidth when connected wrong (because of the capacity).

Pieter
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Old 28th September 2010, 07:28 PM   #8
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On some EDCOR interstage trannies I get poorer response if I hook one end (pri or sec) the 'wrong' way. As if the windings fight each other if they are not hooked up in the proper phase relative to each other. Could this be the case here?

If the tranny u got is for 8k load, isn't it only one side u should load with 8kohms? Since it reflects the impedance. If u load it with 8kohms on both sides, isn't the effective impedance 4kohms? Which actually should give u an even better frequency response, but at the cost of signal attenuation.

My guess is that something aint no right

Helpful eh?
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Old 28th September 2010, 07:30 PM   #9
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Hehe, Pieter and I had the same idea! Only I need longer time to put it down
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Old 28th September 2010, 07:40 PM   #10
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Yes, to amplify this a little; The AC grounds should be connected such that the windings don't fight each other through capacitive coupling. Your diagram doesn't show the polarity or the ground connection between the generator and analyzer. If it's a bifilar IT and your amp or test circuit uses an inverting connection (AC grounds at opposite ends of the winding) then this could be the trouble.

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